Cephalopoda Cuvier, 1797

Pohle, Alexander & Klug, Christian, 2024, Orthoceratoid and coleoid cephalopods from the Middle Triassic of Switzerland with an updated taxonomic framework for Triassic Orthoceratoidea, Swiss Journal of Palaeontology (14) 143 (1), pp. 1-32 : 17-18

Class Cephalopoda Cuvier, 1797

Subclass Orthoceratoidea Teichert, 1967

Order Pseudorthocerida Flower & Caster, 1935

Family Trematoceratidae Zakharov, 1996

Genus Trematoceras Eichwald,1851

Trematoceras elegans ( MÜnster,1841)

Trematoceras boreale Schastlivceva,1986

Trematoceras clarum Schastlivceva, 1986

Trematoceras hikichii Niko et al., 2016

Trematoceras insperatum Schastlivceva, 1988

Trematoceras lytosiphon (Gemmellaro,1904) comb.nov Trematoceras mangishlakense Schastlivceva,1981

Trematoceras solidum Schastlivceva, 1988

Trematoceras subcampanile (Kiparisova,1954)

Trematoceras vulgare Schastlivceva,1981

Trematoceras watanabei Niko & Ehiro, 2020

Genus Paratrematoceras Schastlivceva, 1981

Paratrematoceras shevyrevi Schastlivceva,1981

Paratrematoceras ornatum Schastlivceva, 1981

Paratrematoceras salinarium (Hauer, 1846) comb. nov.

Genus Pseudotemperoceras Schastlivceva, 1986 Pseudotemperoceras pulchrum Schastlivceva, 1986 Pseudotemperoceras nyalamense (Chen, 1981) comb. nov.

Genus Phatthalungoceras Tongtherm & Nabhitabhata,2018 Phatthalungoceras srisuki Tongtherm & Nabhitabhata, 2018

Genus Zhuravlevia Doguzhaeva,1994

Zhuravlevia insperata Doguzhaeva, 1994

Only species with sufficiently known internal characters are listed here.All other species are currently too poorly known to allow for generic assignment and may represent synonyms of other species

Palaeoecology of Triassic orthoceratoids

Considering the absence of soft tissue preservation, interpreting the ecological role of trematoceratids is challenging and remains speculative to some extent. However, the detailed excavations at Monte San Giorgio can at least provide some indirect evidence by revealing distinct distribution patterns. As with their Palaeozoic counterparts, the distribution of trematoceratids appears to be highly facies-dependent, i.e. they are almost entirely restricted to dolomite beds in the middle part of the Besano Formation that represent a deeper, intraplatform basin (Röhl et al., 2001). Teir common association with dasycladacean algae, gastropods and bivalves of the genus Daonella indicates that they inhabited the photic zone. Ammonoids and coleoids usually occur within the same horizons, though it is difficult to discern distinct taxon-specific patterns due to the general abundance of ammonoids and scarcity of other cephalopods. Coleoids appear to be more numerous in the lower Besano Formation (below bed 53), while trematoceratids appear to be more common above this level. However, as it is not always straightforward to distinguish between the two groups, it is possible that these observations bear no significance. Palaeozoic orthoceratoids frequently form mass occurrences (e.g., Bogolepova & Holland, 1995; Hewitt & Watkins, 1980; Kröger & Pohle, 2021; Pohle & Klug, 2018a), suggesting that they may have formed moderate to large shoals, at least during mating season as in many modern squid. Occurrences of Triassic orthoceratoids are much more isolated and thus, they had perhaps a more solitary lifestyle. Tis would also be supported by their comparatively large embryonic conchs, suggesting lower reproductive rates (high survivorships, k-strategy) in contrast to the presumably more prevalent r-strategy in Palaeozoic orthoceratoids (Laptikhovsky et al., 2018). Even if the absolute size of the embryonic shell may seem small, it is very large when compared to the size of the adult animal, which is not known to exceed a couple of centimetres in diameter. Te actual size at hatching is difficult to estimate, but it is probable that it occurred somewhere around the point where the initially high expansion rate stabilised, i.e. approximately 3–4 mm. It is also notable that the initial chambers of the k-strategist Nautilus form at similar whorl diameters (Tajika et al., 2015), even if the embryonic shell itself is much larger (Arnold, 1987; Boletzky, 1988).

Palaeobiogeography and stratigraphic distribution

of Triassic orthoceratoids

During the Triassic, trematoceratids had a global distribution (Fig. 8). To our knowledge, orthoceratoids have not yet been reported from Induan deposits, possibly indicating that they underwent an evolutionary bottleneck during that time. Pseudotemperoceras nyalamense (Chen, 1981) and the co-occurring “ Michelinoceras cf. lytosiphon ” and “? Michelinoceras sp. B ” from the Early Triassic lower Tulong Formation of Tibet represent perhaps the oldest record of Triassic orthoceratoids (Chen, 1981). Te stratigraphy of the Tulong Formation is now better resolved with detailed ammonoid, sedimentary and carbon isotope data (Brühwiler et al., 2009, 2010), though it is unclear from which beds the orthoceratoids were collected. According to Brühwiler et al. (2009), orthoceratoids are common in subunit IVa, which corresponds to the early Spathian and thus the earliest Olenekian. Orthoceratoids are better known from later Olenekian deposits of East Asia (Brühwiler et al., 2009; Kiparisova, 1954; Kummel & Erben, 1968; Niko et al., 2016; Schastlivceva, 1981, 1986, 1988; Shigeta & Nguyen, 2014; Shigeta & Zakharov, 2009), but also from Eastern Europe (Germani, 1997; Grădinaru et al., 2007) and North America (Brayard et al., 2019). In some Olenekian localities, orthoceratoids are rather common, although they have usually not been studied in detail (e.g., Brayard et al., 2019; Brühwiler et al., 2009). It thus appears that despite the short hiatus in the earliest Triassic, trematoceratids recovered relatively quickly and already reached a widespread distribution concentrated in the northern hemisphere during the Early Triassic (although this may be caused by sampling biases; see, e.g., Vilhena & Smith, 2013; Close et al., 2020). Teir range extended from the northwestern margin of Pangaea over arctic regions up to the western and southern Tethys. It is remarkable that trematoceratids have been found in both low and high palaeolatitudes, suggesting that they adapted to a range of climatic conditions already early in the Triassic. Te maximum generic diversity (and thus morphological disparity) within the Trematoceratidae was apparently already reached during the Olenekian, where the three genera Trematoceras , Paratrematoceras and Pseudotemperoceras were already present ( Schastlivceva, 1988). Trematoceratids further flourished during the Middle Triassic, where they are particularly well documented within the western Tethyan realm but have been found in many areas worldwide. While trematoceratids are still relatively well-documented in the Carnian of the Alps, they appear to have had a more restricted range during the Late Triassic. Other known occurrences are from the Carnian of equatorial western Pangaea, i.e. modern day California (Hyatt & Smith, 1905) and from the southeastern coast of Pangaea, i.e. New Zealand and Timor (Bülow, 1915; Trechmann, 1918). Norian orthoceratoids have occasionally been reported although they appear to be much rarer. Tis indicates that the decline of the group started already before the end-Triassic mass extinction event. Tis is further corroborated by the fact that only a single Rhaetian trematoceratid has been reported, from the Zlambach Marl of Austria (Jeletzky & Zapfe, 1967).